An ultrasound system has been extensively used for acquiring internal information of a target object due to its non-invasive and non-destructive nature. Since the ultrasound system may provide a high resolution image in real-time without any surgical treatment, it has proven to be very helpful in the medical profession.
Generally, the ultrasound system provides B mode (brightness mode) that shows a reflection coefficient of a ultrasound signal reflected from a target object in a 2-dimensional image, doppler mode showing an image of a moving target object using doppler effect, motion mode (M mode) showing a change of biometric information (e.g., brightness information) over time of a target object in a particular part of a B mode image, and elastic mode showing a difference of reaction between a target object with and without compression in an image.
Also, the ultrasound system sets an M line on the B mode image and the color doppler mode image, and provides the color M mode image and the brightness M mode image corresponding to the M line. The color M mode image may show how the blood flow changes over time on the M line. Typically, the M mode acquires ultrasound data within a particular time interval (i.e., sweep period) and displays an M mode image formed from the acquired ultrasound data. The sweep speed of the color M mode is commonly used in a range of 60 to 360 Hz, which is not sufficient from the point of the time resolution. In the brightness M mode, assuming that the velocity of sound is 1540 m/s and the depth is 15 cm, the time required to acquire the ultrasound data corresponding to one scan line is very short, i.e., approximately 200 μs, when the pulse repetition frequency (PRF) of the ultrasound is 5 KHz. Thus, the problem of the time resolution does not occur. In other words, the brightness M mode has no problem in acquiring the ultrasound data corresponding to one scan line. The color M mode acquires a plurality of ultrasound data on one scan line by repeatedly transmitting and receiving an ultrasound signal as many times as an average number of the data acquisition (an ensemble number) in the same direction. An ultrasound data acquisition period for acquiring ultrasound data of color M mode is inversely proportional to the sweep speed. For example, if the ultrasound data acquisition frequency (the PRF for radiating the ultrasound) is 2 kHz and the average number of the data acquisition is 12, then the time for forming the color M mode image according to one scan line is 6000 μs=500 μs (the ultrasound data acquisition frequency (2 kHz))×12 (the average number). In such a case, the sweep speed should be less than 160 Hz ( 1/6000 μs). With this low speed, it is difficult to provide continuous color M mode. Also, as the average number is reduced to increase the sweep speed, the ultrasound data for forming the color M mode image should be reduced. This also degrades the quality of the color M mode image.